Dither-quantized signalling for color television

ABSTRACT

One or more of the video components of prior art are quantized coarsely before being sent to a receiver. To prevent perceptible loss of picture information and to minimize visible artifacts, suitable ordered dither is added before a component is quantized. The transmitted signal is preferably also sampled at the transmitter and resampled and requantized in the receiver for combatting noise and distortion. Some forms of the invention are compatible with prior art receivers. Other forms provide compressed digital signalling and coarsely quantized pulse-amplitude modulation.

SUMMARY

This invention concerns the sending and reproduction of coloredtelevision pictures. In television transmitters of the prior art, inputdevices (including color camera and matrix unit) generate componentvideo signals which are substantially replicated in associatedreceivers. Output devices (including matrix unit and display) of aprior-art receiver convert said replicated components into a displayedpicture. My invention utilizes the transmitter-input prior art and thereceiver-output prior art, but alters at least one of the componentvideo signals sent from the transmitter and replicated in the receiver.This is done in a manner which does not appreciably impair the qualityof the perceived picture but facilitates substantially exact replicationof signals and has other advantages.

Specifically, one or more of the video components generated by prior-artdevices are coarsely quantized before being sent to a receiver. Suchcoarsely quantized signals can usually be requantized after reception toremove effects of noise and distortion in the communications channel.However, to prevent the loss of perceptible picture information inconsequence of coarse quantization of the signal amplitude, and tominimize visible artifacts, suitable ordered dither is first added to asignal before it is quantized. The transmitted signal is preferably alsosampled at fixed rate and may also be resampled in the receiver forfurther aid in restoration.

Some forms of my invention are compatible with television transmittersor receivers of the prior art without, however, realizing all of thebenefits of the invention when used in that manner. Other forms providebinary digital signalling between transmitter and receiver, and stillother forms signal by means of a composite signal encoded in quantizedpulse-amplitude-modulated (quantized PAM) form. The latter form issuitable for use with digital privacy or encryption means but does notrequire more than conventional television bandwidth. Binary signallingis accomplished with appreciably less channel capacity than comparablePCM transmission.

OBJECTS OF THE INVENTION

One object of the invention is to transmit a color television picturehaving better quality than has heretofore been possible in relation tothe communications channel and equipment complexity.

Another object of the invention is to combat effects of noise anddistortion introduced in a color television channel.

Another object of the invention is to overcome effects of noise anddistortion introduced by video recording equipment for color pictures.

Another object of the invention is to achieve the above objects inequipment which is also compatible with prior-art transmitters andreceivers.

A further object of the invention is to transmit color televisionpictures by means of compressed binary signalling, using relativelysimple and economical apparatus.

A further object of the invention is to transmit high quality colortelevision pictures by means of M-ary digital signalling, using asubstantially conventional analog channel and a relatively small valueof M.

EXPLANATIONS AND REFERENCES

The expressions "communication channel" and "transmission channel" areused herein to refer to both direct channels (such as radio or wirelinks) and means for recording and later reproduction. Likewise,"transmitter" may refer to a video recording device and "receiver" tothe associated playback equipment.

"Ordered-dither coding" or simply "dither coding" refers to coarsequantization of signal amplitudes subsequent to the addition of ordereddither to the signal. Ordered-dither coding of a monochrome televisionsignal is disclosed in my U.S. Pat. No. 3,739,082, entitled "OrderedDither System", with special reference to 3-dimensional ordered ditherand nasik dither patterns. Although this kind of ordered dither is notessential for the present invention, I generally prefer it, especiallyin conjunction with conventionally interlaced television scanning.

Briefly, a 2-dimensional ordered dither pattern has a rectangular arrayof dither samples repeated horizontally and vertically over the entiretelevision scanning raster, and a three-dimensional dither pattern alsochanges between successive frames of the televised moving picture.

There are a number of literature descriptions of 2-dimensional ordereddither patterns including Lippel and Kurland, "The Effect of Dither onLuminance Quantization of Picture", IEEE Trans. Comm. Technol., COM-19,No. 6 (Dec. 1971) and Bayer "An Optimum Method for Two-Level Renditionof Continuous Tone Pictures", Int. Conf. on Commun., Conf. Record, pp.26-11 to 26-15, 1973. A specific kind of 2 -dimensional ordered ditheris also described in U.S. Pat. No. 3,997,719, entitled "Bi-Level DisplaySystems" and issued to Judice. Three-dimensional ordered dither is alsodescribed in my article "Two-and Three-Dimensional Ordered Dither inBi-Level Picture Displays". Proc. of the S.I.D., vol. 17/2, 2nd Quarter1976. (Although the last three references are principally concerned with2-level quantization, dither patterns are the same with a larger numberof levels.)

When necessary or advisable, the invention will be described in terms ofthe NTSC color system (standard in the United States) and the Y(luminance) and I and Q (chrominance) components of said system. Itwill, however, be clear to persons skilled in the art how the principlesof the invention apply equally to other systems such as PAL and SECAM(which are conventional in other countries) and also to recording andplayback systems.

FIGURES

FIG. 1 shows a transmitter corresponding to an embodiment of theinvention and also compatible with certain prior-art color-televisionreceivers.

FIG. 2 shows the corresponding receiver which is also compatible withcertain prior art transmitters.

FIG. 3 shows a transmitter embodiment which sends a binary digitalsignal to its receiver, and FIG. 4 is the receiver.

FIG. 5 shows a transmitter embodiment of the invention which transmitscolor pictures to a receiver by means of quantized positive and negativepulses, and FIG. 6 is the corresponding receiver.

FIG. 7 depicts one example of an ordered dither generator of the priorart suitable for use in my invention.

FIG. 8 shows certain ideal waveshapes associated with FIGS. 3 and 4.

FIG. 9 shows certain ideal waveshapes associated with FIGS. 5 and 6.

EMBODIMENT OF FIGS. 1 AND 2

FIG. 1 represents the transmitter and FIG. 2 the receiver of a prior-artcolor television system which has been improved in accordance with oneembodiment of the invention. According to only prior art, camera unit 1of FIG. 1 generates primary-color video signals R, G and B (for red,green and blue) which are converted in matrix unit 2 into luminancesignal Y and chrominance signals I and Q. Low-pass filters 12 and 13have been included to indicate that I and Q have less bandwidth than Y.Horizontal (H) and vertical (V) scanning signals are provided by thesweep unit 3 which also provides synch signal 8 for both horizontal andvertical synchronization. In the prior art, Y, I, Q, synch 8, and anaudio input 10 are all combined in encoder unit 6. The output from 6 isa composite signal 16 containing substantially all the information putinto the encoder. The composite signal 16 is furnished to channel unit7, which may be a broadcast transmitter, recording device, or the like.

Units added in accordance with this embodiment of the invention includedither generating circuits 9 attached to the sweep unit 3, summing unit4, sampler 14, and quantizer 5. The Y signal is not furnished directlyto the encoder, but to summer 4 instead, where it is added to a dithersignal 11 from dither generator 9. Units 3 and 9 also furnish a clocksignal 15 (assumed to be at 8×10⁶ pulses per sec.) which samples theoutput from summing unit 4. The resulting samples are quantized into Ndiscrete levels by quantizer unit 5. Then the stream of quantizedsamples, designated Y', is furnished to encoder 6 in place of the Ysignal of prior art.

One suitable form of dither generator 9 is disclosed generally in myU.S. Pat. No. 3,739,082 and, furthermore, a particular arrangementsuitable for use with NTSC scanning is discussed below with the aid ofFIG. 7. The quantizer 5, which is not itself part of the invention, canbe constructed in various ways. It preferably has a "staircase"input-output characteristic with N steps, such as can be produced bycombining outputs from N-1 diode circuits or bistable amplifiers, biaseddifferently so that they trigger "on" at progressively higher amplitudevalues of a signal applied to all inputs simultaneously. The prior-artsumming unit 4 and sampler 14 are so well known as to require no furtherdiscussion. The value of N in the quantizer is typically between threeand eight, but two steps or more than eight are also feasible.

FIG. 2 represents a color receiver suiable for the transmitter ofFIG. 1. Channel unit 27 (which may be a radio-signal receiver, aplayback unit for video recordings, or the like) supplies compositesignal 16 of the transmitter to prior-art decoder 26. The decoderseparates out the various component signals of the composite, and H andV sweep signals are generated to produce a scanning raster in displayunit 21 in synchronism with the raster of camera 1 of the transmitter;all this is according to prior art. Further in accordance with priorart, Y, I and Q components (corresponding to the output of transmittermatrix 2) would be recovered by decoder 26 and collectively converted inreceiver matrix unit 22 into the primary-color video signals R', G' andB' which, in turn, produce a color picture in prior-art display unit 21.According to the instant invention embodiment, however, the dither-codedluminance signal Y', and not the original luminance signal Y, isincorporated into the composite signal and recovered (along with I andQ) by decoder 26. I prefer to resample Y' in optional resampling unit 28substantially in synchronism with clock pulses 15 of the transmitter.According to the invention, the Y' signal is requantized in quantizerunit 25 which has steps corresponding to those of the transmitterquantizer 5. Requantization of the recovered Y' signal (preferablyaccompanied by resampling) removes unwanted noise and distortion whichmay have been added during transmission from the transmitter. Theregenerated Y' signal is then furnished to matrix unit 22 together withI and Q from the decoder and the three signals are thereafter processedand displayed like the Y, I and Q signals of prior art.

If encoder 6, decoder 26, channel units 7 and 27 and the interveningcommunications channel all provide sufficiently generous bandwidth inthe Y' channel in relation to the sampling ratio, the recovered Y'signal may have detectable pulsations (due to the sampler 14), ontowhich a highly stable oscillator can be locked in accordance with priorart for preferred resampling of Y' in the receiver. In the manyinstances, when it is not practicable to provide enough bandwidth forthis purpose, other known methods can provide a suitable clock signal115. For example, an independent clocking oscillator can be phase-lockedto the horizontal synch or to a special "clock-burst" signal, similar tothe "color-burst" of th NTSC system; in some cases, it may even bepossible to use the color burst itself, or the color carrier, forsynchronization.

An especially simple and accurate method for reclocking in the receiveris practicable when channel units 7 and 27 are recording and playbackdevices, viz., to record the clock signal on a separate track and toreproduce it along with the composite video. The dotted connections 15'on FIGS. 1 and 2 represent such recorded clock signals and it should beunderstood that 15' of FIG. 2 replaces the independent clock signal 115mentioned above. It will no doubt occur to persons skilled in the artthat the same recorded clock signal can be employed to overcome effectsof variations in recording and playback speed. In the particular case ofdisk records, only one circular track is needed for the clock signal,while the composite video signal can be recorded on a continuous spiralof much greater length.

Although FIGS. 1 and 2 show only the luminance component Y to bedither-coded, it will be evident to persons skilled in the art thateither or both chrominance components can be dither coded andregenerated in similar fashion without departing from the spirit of theinvention. Inasmuch as much more pictorial information is included inthe luminance component than in the chrominance components, and inasmuchas the human psychovisual system is most sensitive to noise anddistortion in the Y channel, quantization and regeneration of Y is moreimportant than corresponding protection of the I and Q components.Furthermore, for communications efficiency, a chrominance signal shouldbe dithered and sampled at lesser rate than Y, and quantized into fewersteps. This is explained more below, in connection with otherembodiments of the invention.

If the transmitter of FIG. 1 corresponds to one of the prior art exceptfor sampling and dither coding of the luminance component, a compatibleprior-art receiver linked to the transmitter of FIG. 1 will display thesame color picture as the receiver of FIG. 2 without, however,counteracting noise and distortion.

EMBODIMENT OF FIGS. 3 AND 4

A second general embodiment of my invention is characterized by dithercoding of all video components, use of analog-to-digital converters(a/d's) to quantize in the transmitter, and multiplex transmission ofthe resulting binary digits (or their digital equivalents) in accordancewith well-established prior art. A receiver demultiplexes and decodesthe transmitted digital signal, to reconstruct separate luminance,chrominance and synch signals from which a prior-art display unitreconstructs the colored television picture. As a general rule, sincethe chrominance channels have less bandwidth, they are sampled lessfrequently than the luminance channel which has more bandwidth, and theyare also quantized more coarsely, i.e. with fewer bits per sample.Therefore, the number of bits per second transmitting luminance isusually much greater than the number of bits per second transmittingeither chrominance component.

FIGS. 3, 4 and 8 refer to a particular example of such generalembodiment. For the system of these figures, the Y bandwidth is assumedto be 4 MHz., the I bandwidth 1.5 MHz. and the bandwidth of Q 0.5 MHz. Yis, therefore, sampled 8×10⁶ times per second, but each of I and Q atonly half that rate. Furthermore, Y is digitized with two binary digits(bits) per sample, corresponding to four quantized levels, but I and Qhave only one bit per sample, corresponding to two levels.

The part of FIG. 3 enclosed in a dotted rectangle and designated 100 ismerely color-input equipment of the prior art, including camera 1,raster and synch generator 3 and matrix 2 which were previouslydiscussed in relation to FIG. 1. Low-pass filters 31 and 32 have beenincluded to suggest the lower bandwidths of the two chrominancechannels. A dither generator 44 uses signals from the raster generator 3to generate one dither signal DY for the Y channel and, in this example,another dither signal DI which goes to both I and Q channels; with otherexamples, there may be a separate dither generator for each videocomponent. Dither is generated in accordance with prior art, alsodiscussed later herein. Clock signal 215 is assumed in this example tohave a rate of 24×10⁶ pulses per second and is brought out from unit 44.

Ignoring, for the present, delay unit 30, the Y output of matrix 2 isadded to DY in unit 33 and their sum presented at the input ofanalog-to-digital converter 36 (a/d 36). Likewise, the sum of I and DIappears at the input to a/d 37 and the sum of Q and DI at the input to38. The analog-to-digital converters 36, 37, 38 and 210 and multiplexingunit 39 are incorporated into prior-art encoder and multiplexer 101,shown enclosed within a dotted rectangle. Both 101 and the complementarydemultiplexer-decoder 102 of FIG. 4 are entirely of the prior art asdiscussed, for example, in the book "Transmission Systems forCommunications", published by Bell Telephone Laboratories (4th edition,1970), with particular reference to Section 6.3 and Chapters 24, 25 and26. They will, therefore, not be described in further detail.

Returning to FIG. 3, the audio signal 10 may go, if desired, to anothera/d designated 210. The synch signal 8 is furnished directly tomultiplexer 39. Under control of clock signal 215 and synch 8,encoder-multiplexer 101 samples the various inputs, digitizes thesamples into binary form, and combines the binary outputs into a singlebit stream 43. In the present instance, since 37 and 38 are only one-bita/d's, they are realized by means of 2-level quantizers (hard clippers)whose output is sampled by the interrogation pulses 41 and 42 to obtain1-bit samples on leads 46 and 47 respectively. On the other hand,interrogation 40 causes a 2-bit word from a/d 36 to be sent back on lead45.

FIG. 8 represents waveforms and sampling during scanning of part of aline of the television raster to explain details of the multiplexing.Both luminance and chrominance are assumed to be constant during thesmall scanning interval shown. Dither signal DY is seen to be arepeating function 151. Although DY changes in waveform from line toline and frame to frame, it is merely repeated over and over on a singlescan line, as indicated, the amplitude changing 8×10⁶ times per secondso three clock periods occur during each fixed-amplitude interval. A/dconverter 36 is interrogated once during each such interval and producestwo bits which are then transmitted by the multiplexer in synchronismwith the clock pulses at times labelled Y₁ and Y₂ on the waveform.Meanwhile, dither generator 44 has also put out dither signal DI, havingwaveform 152. The amplitude changes in 152 half as often as in 151, thatis to say, once every six clock pulses. The sum I+DI is sampled 4×10⁶times a second and a single bit representing its 2-level quantized valueis inserted into the bit stream 43 at a time (shown by I) correspondingto a clock pulse without a Y bit. Finally, the sum Q+DI is sampledmidway between I samples and quantized with one bit, and the bitinserted in the bit stream at places marked Q. The combined effect is astream of interleaved Y, I and Q bits, as shown at the bottom of FIG. 8which also shows a synch message labelled SS . . . SS. It will beunderstood that the message SS . . . SS . . . occurs during an intervalbetween scanning lines.

Under control of synch signal 8, encoder-multiplexer 101 ceases tointerrogate unit 36, 37 and 38 during the horizontal blanking periodfollowing a line scan and transmits horizontal and vertical synchmessages of the prior art during this interval. In the assumed NTSCsystem, the blanking intervals recur 15,750 times per second. Therefore,audio input 10, digitized by a/d 210, can, if desired, also betransmitted during the blanking intervals in accordance with prior artnot part of the invention.

Channel unit 7 of FIG. 3 and channel unit 27 of FIG. 4 are of the priorart and provide bit stream 43 to demultiplexer-decoder 102 of FIG. 4,already resampled and requantized into distinct binary signals tominimize effects of noise and distortion. Unit 102 detects the synchmessages of the bit stream which then serve, in accordance with priorart, to synchronize demultiplexer 50 with multiplexer 39 of thetransmitter and also to synchronize the horizontal and vertical sweepsof raster scan generator 57 with corresponding H and V sweeps of camera1 of the transmitter. Prior-art demultiplexer-decoder 102 is enclosed bya dotted rectangle on FIG. 4. Demultiplexer 50 of unit 102 distributesseparate component signals in binary form onto leads 45, 46 and 47respectively. The 2-bit words representing Y+DY samples are decoded bydigital-to-analog converter 51 (d/a 51) into 4-levelpulse-amplitude-modulated (PAM) pulses which appear at the output 52.The 1-bit I+DI and Q+DI signals are arranged, in accordance with priorart, so that they also correspond to 2-level PAM samples. Lead 46 is,therefore, shown connected directly to prior-art flip-flop unit 55 whichstores an output I' until a new sample appears on lead 46; likewise,lead 47 is connected to flip-flop 56 which stores output Q' in the samefashion. If necessary, d/a 53 may be used in accordance with prior artto provide an audio output 110, as indicated on the drawing. Theflip-flop units insure essentially continual I' and Q' inputs to matrixunit 22 during the entire period of an active scan line. Meanwhile, a Y'input to matrix 22 is provided from d/a 51 (through delay unit 54 whichwill be discussed presently). Hence Y', I' and Q' component signals(which are dither-coded representations of Y, I and Q of the prior art)appear simultaneously at the inputs to matrix unit 22, a part of theprior-art output display unit 103. Y', I' and Q' are converted intoequivalent primary-color components R', G' and B' and displayed as acolor picture as in the prior art. Ordinarily, at least unit 103 willnot have sufficient bandwidth to follow the 8×10⁶ pulses per second rateof the Y' pulses and the effect will be as if Y' also were storedbetween samples; but even otherwise, the consequent dot pattern producedon the displayed picture would not be more serious than the horizontalscanning lines of prior art.

Storage of the I' and Q' component signals in flip-flop units 55 and 56effectively delays them relative to Y'. Therefore, it is desirable thatY-channel compensating delays 30 of FIG. 3, 54 of FIG. 4, or both, beinserted as shown. This is in addition to other channel compensationdelays which may be provided in accordance with prior art and which arepreferably adjusted for best register of the R', G' and B' components ofthe picture ultimately displayed.

It is conventional in the art to consider that each sample of theluminance component Y defines a picture element (pel). It will beapparent to those skilled in the art, therefore, that the particulardetails used for description of the FIGS. 3 and 4 embodiment of myinvention correspond to binary encoding of color television using 3bits/pel and a bit rate of 24×10⁶ bits per sec. It will also be apparentthat, in general, any of the Y, I, and Q components could have beendigitized with one or more bits (instead of exactly two for Y and onefor the other component signals) and other sampling rates could alsohave been used, to obtain other values of bits per pel and bits per sec.in the combined signal 43 and/or different apportionment of thetransmitted information among the luminance and chrominance components.

EMBODIMENTS OF FIGS. 5 AND 6

FIGS. 5 and 6 show yet another way in which my invention can be used.For the sake of increased clarity, the drawings show only, first,processing of previously described Y, I, Q, DY, DI, DQ signals and aclock signal to produce in the transmitter a composite signal 75 and,second, processing of the equivalent composite 175 in the receiver torecover Y', I' and Q' approximations of Y, I and Q and to display thecorresponding television picture in the manner already described forother embodiments.

The Y, I and Q input video signals of FIG. 5 are the same as those ofFIG. 3. Also the unit 44 shown in FIG. 3 generates DY, DI and DQ dithersignals and furnishes a clock signal 249 which, in this instance, is at8×10⁶ p.p.s. (pulses per sec.). Although the same dither may optionallybe employed in both I and Q channels, different DI and DQ will beassumed, for greater generality. A one-stage binary counter 60 dividesthe clock pulses 249 into one pulse stream 61 corresponding toodd-numbered clock pulses and another pulse stream 62 corresponding toeven pulses. Both new pulse streams are, therefore, at 4×10⁶ p.p.s., butdisplaced from each other. Summing unit 33 combines luminance signal Ywith dither DY. The sum is then sampled in unit 63 in synchronism withclock pulses 249 and then quantized into N levels, by means of unit 66which is similar to the quantizers of FIGS. 1 and 2. Typically, N isbetween 3 and 6. Since Y has only one polarity, in accordance with theprior art, the dither coded output Y' will be taken to be alwayspositive. Y' is presented to multiplier unit 74 on input 69.

Further in accordance with prior art, the chrominance component signalsI and Q are bipolar; that is to say, they vary from positive tonegative. I and DI are added in unit 34, sampled in unit 64 insynchronism with the odd pulses 61, and quantized into uniform positiveor negative pulses I' in 2-level quantizer 67. Q and DQ are likewiseadded in summing unit 35, sampled in synchronism with even pulses 62,and quantized into uniform positive and negative pulses Q'. By means ofOR circuit 72 of the prior art, the I' and Q' pulses are merged into asingle stream (consisting of bipolar pulses at 8×10⁶ p.p.s.) which goesto a second input 73 to the multiplier unit 74. The latter is anysuitable device of the prior art, such as an analog multiplier or aswitching arrangement for reversing the polarity of an amplifier outputwhen the input on 73 changes sign.

It will now be clear to persons skilled in the art that the stream ofoutput pulses 75 from multiplier unit 74 has been quantized into Npositive and N negative levels (2N in all); that the magnitude of apulse corresponds to Y' (the dither-coded representation of Y); and thatthe algebraic sign corresponds to either I' (the dither codedrepresentation of I) or Q' (the dither-coded representation of Q),according to whether the pulse count is odd or even. Although not shownon FIG. 5, it will be abundantly clear to persons skilled in the art howsynch signals can be transmitted from time to time intermittently, toinsure that proper odd and even parity of the clock pulses can bedetermined in a receiver; and it will also be clear how the same synchsignals can provide horizontal and vertical synchronization of rasterscanning. However, when the combined signal 75 is recorded for laterplayback, I prefer to also record a timing track synchronized with oneof the outputs of counter 60, say the odd output 61.

Referring now to FIG. 9, the sum Y+DY is shown at (a), along with therepresentation thereof, Y', which is seen to be sampled (at the rate ofclock signal 249) and also quantized (five different levels shown). At(b) is shown I+DI and its representation I'. I' is synchronous with oddsamples of Y' and quantized into uniform positive and negative pulses.Likewise, (c) shows Q×DQ; and Q', wherein the pulses are synchronizedwith even pulses of Y'. The merged I' and Q' pulses are shown at (d),and at (e) we see the composite signal 175 having pulse amplitudescorresponding to (a) and the sign of each pulse corresponding to (d). Atransmission channel which has enough bandwidth for the original Yinput, but not enough to define individual pulses, will deliver theenvelope 175', shown dotted.

In the receiver of FIG. 6, it has been assumed that the odd clock pulses61 are recovered from a record timing track at the same time that thecomposite signal 175 is reproduced from a record. Said pulses aredelayed one-eighth microsecond in unit 184, as shown, to obtain areplica 62' of the even clock pulses. Combining 61 and 62' in OR circuit80, we also obtain a replica 249' of the 8×10⁶ p.p.s. clock pulses 249of the transmitter. In some systems, where it may not be practicable tofurnish the clock signal 61 at the receiver input, an accurate localclock oscillator may be phase-locked, in accordance with prior art, tothe horizontal synch pulses (not shown in FIGS. 5 and 6); approximately250 pulse oscillations occur in clock signal 61 during the intervalbetween horizontal synch pulses. Other prior-art ways to obtain thethree clock signals will also occur to persons skilled in the art.

Composite signal 175 is rectified in full-wave rectifier unit 80 andthen sampled with clock signal 249' in sampler 83 to recover the Y'pulses. For best use of the invention, I prefer to also requantize Y' inunit 86 which has quantizing steps corresponding to unit 66 of FIG. 5.The Y' output is additionally shown at (f) of FIG. 9. The bipolarcomposite 175 is also sampled in two samplers 84 and 85, preferablyafter positive and negative clipping in optional 2-level quantizer 81.Under control of the odd pulses 61, unit 84 sorts out the I' samples andflip-flop 87 stores the samples between sample periods. FIG. 9 (g) showsthe samples I' by heavy lines and I", the continuous output of theflip-flop, by a lighter line. In the same manner, sampler 85, undercontrol of clock 62', sorts out Q' pulses and flip-flop 88 puts out acontinuous signal Q", shown in FIG. 9 (h). Inasmuch as the I" and Q"outputs are delayed by one-eighth microsecond relative to the I' and Q'samples, broadband delay unit 89 is preferably included to delay the Y'output a like amount; the delayed signal Y" is shown at FIG. 9 (i).

The Y", I" and Q" video signal components are converted into a displayedtelevision picture by means of the prior art equipment shown as 103 inFIG. 4. Details of the conversion process, using the luminance andchrominance signals together with synch signals not shown in FIGS. 5 and6, are well known to the art and have already been discussed herein.

Certain simplifications and economies can usually be effected in thepractical implementation of the system shown schematically in FIGS. 5and 6. For example, instead of using two quantizers, 67 and 68 and ORcircuit 72, the odd and even chrominance samples from units 64 and 65could be combined in a summing amplifier, so that a single quantizerwould serve for both I' and Q'. Furthermore, if multiplier 74 isimplemented by means of a polarity-controlled switching circuit, it maybe feasible to omit the quantizer, or to merely compress the pulseaplitudes supplied to the input 73.

It will be clear to persons skilled in the art that the pulses of thecomposite signal 75 can also be coded in other ways without departingfrom the spirit of the invention. For example, it can easily be arrangedthat the I'+Q' pulses vary between zero and the (N+1) quantizer level,instead of either side of zero, and multiplying unit 74 can be replacedby a summing amplifier. The output pulses 75 will then have 2N+1possible quantized values; values greater than N signify the sameinformation as a positive pulse in FIG. 9 (e) and values of N or lesscorrespond to a negative pulse. In the former case, Y' corresponds tothe difference between the pulse level and N+1, in the latter case, Ycorresponds to N.

GENERATION OF THE ORDERED DITHER SIGNALS

Ordered dither suitable for this invention has been described in theliterature and also in my U.S. Pat. No. 3,739,082. An ordered dithersignal is a repetitive sequence of pulses having various specifiedamplitudes, corresponding to a horizontally and vertically repeatedpattern of dither samples in relation to the television scanning raster.The distribution of pulse amplitudes is preferably adjusted so that thepeak-to-peak deviation is slightly less than the interval betweenquantizer steps (assumed to be the same for all steps); however, with a2-level quantizer, the range of values is sometimes made more or less,to vary the contrast.

One technique for generating an ordered dither signal is bydigital-to-analog conversion of a suitable sequence of binary numbersobtained by logical combination of binary signals (square waveoscillations) derived from the rasterscanning circuitry of thetransmitter. This method will presently be shown by example. Otherequivalent methods include readout from storage, in synchronism with thescanning, and the like.

Although not essential for the invention, I prefer 3-dimensional nasikdither of the referenced patent. FIG. 7 shows a practical arrangementfor generating only the DY dither signal of FIGS. 3 and 5 and DI and DQwill be discussed later. The arrangement of FIG. 7 follows the teachingof the prior patent but particular logic has been worked out to utilizethe odd number of scanning lines and interlaced scanning of the NTSCsystem, and also to make use of convenient hardware devices.

Units 300 through 303 inclusive are identical parity-check units of theprior art, similar to commercially available units. Each such unit has anumber of inputs 309 whereby binary information can be inserted (sixinputs per unit shown on the drawing). One specified signal levelcorresponds to a 1, and another specified level to a 0, following theconventional art. A square-wave signal which alternates betweenappropriate amplitude values therefore corresponds to alternate 0's. Itis characteristic of the parity check units 300-303 that the output ofunit 300, for example, is 1 when an odd number of 1's is present on thesix inputs and 0 when the an even number is present. (An input to whichno connection is made is considered to receive 0.)

The NTSC scanning raster has 525 lines with 2:1 interlace, horizontalscanning frequency of 15750 Hz. and frame rate of 30 frames (60 fields)per sec. Also, we have consistently assumed a luminance sampling rate of8×10⁶ pels/sec. In synchronism with the horizontal scanning, wetherefore provide a square-wave signal, designated A, having frequencyof 7875 Hz. (half the scan frequency) and connect A to inputs of units302 and 303, as shown. From the 60 Hz. vertical sweep, we derive squarewaves B and F. B is 30 Hz. and is connected to inputs of 300, 301 and302, while F is 15 Hz. and connected to 300, 302 and 303, as shown. Fromthe clock are derived a 4 MHz. signal D, which goes to 300, 302 and 303and a 2 Mhz. signal C, which goes to 300 and 301. The relative phaserelationships between various square waves is not important.

It will be apparent that four binary outputs (W,X,Y and Z of thedrawing) change continually as the several inputs change. Thecontinually-changing binary number WXYZ is therefore decoded indigital-to-analog converter 304 to obtain dither signal DY. In thisexample, DY has 16 possible amplitudes, which preferably are adjustedfor equal positive and negative peaks, and a peak-to-peak difference of15/16 quantizer step. It will be seen that Z could be omitted, resultingin only eight amplitudes; in such case the optimum peak-to-peak value is7/8 step. Other 2- and 3-dimensional nasik patterns having 16 steps, aswell as non-nasik ordered dither patterns can be obtained by suitablyaltering the connections to parity-check units. For generating more than16 amplitudes (rarely advisable), it is necessary to have more outputvariable. For 32 amplitudes, a suggested fifth variable is obtainable bydividing both the input signal C and the output signal Z by two, andthen taking the EXCLUSIVE-OR sum.

Inasmuch as the eye is much less sensitive to chromaticity differencesthan to luminance variations, DI or DQ does not need as many amplitudevalues as DY. I may therefore use only the variables W, X and Y, or evenonly W and X. Also, odd-even sampling means that only alternate pels ofa chrominance component are sampled along the horizontal scan line whilefull vertical resolution is maintained. I therefore prefer to strctchthe dither pattern horizontally by a factor of two. This is done byhalving the frequencies of the C and D square waves of FIG. 7. To makeDQ different from DI, I employ a second digital-to-analog converter inparallel and negate one of the binary inputs to said second converteronly.

I claim:
 1. A television transmitter includingmeans for resolving acolored input picture into plural component pictures; means forrepresenting each component picture by a plurality of video signals, theamplitude of each signal varying substantially in proportion tocomponent intensity in successive picture elements arranged in apredetermined scanning sequence; means for combining one of said videosignals with predetermined dither to obtain a first sum; means forapproximating with plural quantizing levels separate successive samplesof the first sum; means for combining another video signal withpredetermined dither to obtain another sum; means for approximating withtwo quantizing levels samples of said other sum; and means forgenerating and transmitting quantized composite samples wherein themagnitude of a composite sample corresponds to an approximated sample ofthe first sum and its algebraic sign corresponds to an approximatedsample of the other sum.
 2. The transmitter of claim 1 wherein thecolored picture is resolved into luminance and chrominance componentsand the video signal corresponding to luminance is the video signal ofthe first sum.
 3. The transmitter of claim 1 wherein the video signalcorresponding to luminance is included in samples having more than twolevels and a video signal corresponding to chrominance is included insamples having two levels.
 4. The television transmitter of claim 1arranged so that one pattern of dither samples is added during someframes of the television picture and another pattern is added duringother frames.
 5. The television transmitter of claim 4 employingthree-dimensional ordered dither.
 6. A television system includingthetransmitter of claim 1, arranged to represent said composite samples bypulses quantized with N levels; a channel for transmitting said pulseswith N amplitude levels, and a television receiver arranged to display acolored picture having a component picture reconstructed from saidquantized pulses.
 7. Means for transmitting plural component signals ofcolor television by discrete pulses includingmeans for sampling a firstcomponent signal to obtain a series of primary pulses having likepolarity relative to a reference; means for sampling a second componentsignal to obtain a series of secondary pulses, some positive and somenegative relative to a reference; means for pairing the secondary pulseswith at least some of the primary pulses; means for generating an outputpulse corresponding to each pair, the output pulse magnitudecorresponding to that of the primary pulse and the polarity having onealgebraic sign when the secondary pulse is positive and the oppositesign when said secondary pulse is negative; whereby, in the series ofoutput pulses, successive polarities represent the second componentsignal quantized with only two amplitude levels and successivemagnitudes represent the first component signal.
 8. The transmittingmeans of claim 7 including also means for sampling a third componentsignal to obtain a tertiary series of pulses similar to the secondaryseries and means for interleaving secondary and tertiary pulses into asingle series;wherein said means for generating an output pulse operateswith primary pulses and interleaved pulses; and whereby successivepolarities in the series of output pulses represent the second and thirdsignal components interleaved and successive magnitudes represent thefirst signal component.
 9. The transmitting means of claim 8 whereinsaid first component signal represents picture luminance and the secondand third component signals represent chrominance components.
 10. Thetransmitting means of claim 9, includingmeans for combining a componentsignal representing chrominance with ordered dither, so that theinterleaved secondary and tertiary series of pulses include said dither;and means for quantizing into discrete amplitudes primary pulsesrepresenting luminance.
 11. Transmitting means according to claim 7,including means for combining a video signal with ordered dither toobtain any of the plural components.
 12. Transmitting means according toclaim 7, including quantizing means for restricting the magnitude of anoutput pulse to discrete values.
 13. A video record having storedthereon the plural component signals transmitted by the means of claim 7or claim
 10. 14. Means for recovering separately a luminance signal andtwo chrominance signals from a series of positive and negative pulsescharacterized by successive magnitudes representing said luminancesignal, by successive polarities in one interleaved fraction of pulsesrepresenting one chrominance signal, and by successive polarities inanother interleaved fraction representing the second chrominance signal;comprisingrectifier means removing chrominance effects from theluminance component represented by pulse magnitudes; two-levelquantization means removing luminance effects from the interleavedchrominance signals represented by pulse polarities; and demultiplexingmeans separating the interleaved quantized chrominance signals from eachother.
 15. The signal recovery means of claim 14, including quantizingmeans for restricting to discrete amplitudes the luminance componentrepresented by pulse magnitudes.
 16. An improved recording and playbacksystem for color television includingmeans for representing thetelevision signal by plural signal components including a luminancecomponent and at least one chrominance component; means for modifying atleast the luminance component by adding ordered dither; means forpulse-amplitude modulating the sum of said luminance component and saiddither and for restricting the pulse amplitudes to plural discretelevels whereby said pulses are quantized; means for representing achrominance component, including any dither modification thereof, byquantized samples restricted to two values of amplitude; means forcombining quantized pulses representing luminance with quantized samplesrepresenting chrominance in composite pulses wherein the magnitude of acomposite pulse represents a luminance sample and the algebraic signrepresents a chrominance sample; means for recording said compositepulses and for playing back the represented signal components; means forrequantizing samples of the played-back luminance component to reducepossibly accumulated noise and distortion; and means for displaying thecolor television picture corresponding to the played-back signalcomponents, including the requantized luminance component.
 17. Thesystem of claim 16 arranged to also record sufficient information for aspecified train of clock pulses and arranged both to resample theplayed-back components with said clock pulses and to requantize them.18. A recording system for color television comprisingmeans forrepresenting a color television signal by plural components, including aluminance component and a chrominance component; means for modifying atleast the luminance component by adding ordered dither; means forrepresenting the modified luminance component by quantized samplesrestricted to several discrete amplitudes; means for representing achrominance component, including any modifications due to dither, byquantized samples restricted to two values of amplitude; means forproducing a composite signal corresponding to positive and negativepulses wherein the magnitude of a pulse corresponds to a quantizedluminance sample and the algebraic sign corresponds to a quantizedchrominance sample; and means for storing said composite signal on amedium to produce a record.
 19. An improved record having stored thereona composite signal comprising luminance and chrominance signalssufficient for playback of color television;wherein the improvementconsists of storing a luminance signal restricted to several discreteamplitude levels and a chrominance signal restricted to two amplitudelevels, at least one of said signals including ordered dither; thestored signal corresponding to a series of positive and negative pulseswherein the magnitude of a pulse represents a luminance sample quantizedwith several possible amplitudes and the algebraic sign represents achrominance sample quantized with two possible amplitudes.
 20. Therecord of claim 19 also storing clock-pulse information suitable forsynchronous resampling of the amplitude-modulated pulses duringplayback.
 21. A receiver for plural video signals transmitted as asequence of composite samples restricted to specific levels ofamplitude; comprising:means for generating a sequence of binary samplescorresponding to algebraic sign in a sequence of said composite samples;means for generating a sequence of other samples restricted to aplurality of specific values corresponding to quantized absolutemagnitudes in a sequence of said composite samples; and means fordisplaying a color television picture including a picture componentcorresponding to said binary samples and a picture componentcorresponding to said other samples.